Microwave oven power supply circuit having hot-wire relays

ABSTRACT

A microwave oven power supply circuit includes a transformer having a primary, high voltage secondary, and a tapped low voltage secondary winding. The start side of the high voltage secondary winding is grounded and the other side of the high voltage secondary winding is connected in series with a capacitor, the normally open contacts of a second hot wire relay, and a diode to ground; with the negative polarity diode terminal facing ground. A magnetron includes an anode connected to ground and a filament connected in series between the tap on the low voltage secondary winding and one side of the low voltage secondary. A connection is made between the positive polarity diode terminal and the magnetron filament. The heater coil of a first hot wire relay is connected across the low voltage secondary winding. Additionally, a set of normally open contacts in the first hot wire relay and the heater coil of the second hot wire relay are connected in series across the low voltage secondary winding.

United States Patent [72] Inventor Benjamin V.Valles Wllliamsport, Pa.[21] AppLNo 799,715 [22] Filed Feb. 17,1969 [45] Patented July 6,1971[73] Assignee Litton Precision Products, Inc.

San Carlos, Calll.

[54] MICROWAVE OVEN POWER SUPPLY CIRCUIT HAVING HOT-WIRE RELAYS 2Claims, 1 Drawing Fig. [52] U.S.Cl 315/104, 2l9/l0,55,328/8,328/270 [51]lnt.Cl H05b41/36, HOSb 1/02 [50] Field otSearch 328/270,8; Y3l5/l04,94;219/481,482, 485, l0.55, 10.75

[56] References Cited UNITED STATES PATENTS l,893,223 1/1933 Burkle315/104X 2,068,883 1/1937 Klinkhamer. 315/104X 2,085,642 6/1937 DeJong3l5/104X 2,227,747 1/1941 Clark 3,225,302 'l2/l965 Ojelid 3,392,3097/1968 Hickman ABSTRACT: A microwave oven power supply circuit includesa transformer having a primary, high voltage secondary, and a tapped lowvoltage secondary winding. The start side of the high voltage secondarywinding is grounded and the other side of the high voltage secondarywinding is connected in series with a capacitor, the normally opencontacts of a second hot wire relay, and a diode to ground; with thenegative polarity diode terminal facing ground. A magnetron includes ananode connected to ground and a filament connected in series between thetap on the low voltage secondary winding and one side of the low voltagesecondary. A connection is made between the positive polarity diodeterminal and the magnetron filament. The heater coil of a first hot wirerelay is connected across the low voltage secondary winding.Additionally, a set of normally open contacts in the first hot wirerelay and the heater coil of the second hot wire relay are connected inseries across the low voltage secondary winding.

PATENTEU JUL 6 IBTI MICROWAVE OVEN POWER SUPPLY CIRCUIT HAVING HOT-WIRERELAYS This invention relates to microwave ovens and, more particularly,to a novel microwave oven power supply having simple and effectiveconstruction to ensure reliable operation and long oven life.

Microwave ovens use high frequency or microwave frequency'energy to heator cook foodstuffs. Contemwary ovens employ the conventional type ofelectron tube termed the magnetron for generating microwave energy. Inturn, the energy generated by that source is passed from the magnetronoutput through suitable microwave passages or waveguide into the cookingchamber or oven cavity, as may be variously termed. The magnetron isconnected to a power supply which in turn is connected to the house orline current. The power supply coriver'tsthe line voltage into both thehigh voltage necessary for themagnetron anode and the low voltage outputnecessary to energize the magnetron filaments. Suitable controls andinterlocks are provided in conventional ovens to regulate theapplication of power to the magnetron. One such control is the interlockwhich prevents the application of power from the power supply to themagnetron should the oven door be open. Other controls include theon-off switch and timer controls which permit the completion of thepower supply circuit for the desired cooking interval. Further, forms ofinternal protection are provided to prevent permanent damage to the ovencomponents, such as fuses, or to ensure proper operation of thecomponents so as to enhance component life, such as a filament highvoltage power sequence circuit.

The latter circuit ensures that the magnetron filaments are heated withsuitable current from a low voltage source for a predetermined intervalprior to the application of high voltage to the magnetron anode. Thisavoids application of highvoltage to the magnetron while the filament iscold preventing internal arcing or operation detrimental to themagnetron or components.

Heretofore this sequencing circuit has been used inoven power suppliesin various forms but complicated and expensive devices, such as cammotor switches, electromagnetic relay, etc., have been used for thispurpose. In operation the sequencing has been functionally independentof the actual filament winding and has generally required the use of aseparate filament transformer. Should the filament winding of thetransformer in such circuit or the filament winding in a separatefilament transformer in the general circuit develop a high resistancewhich lowers the filament voltage and current below recommended levels,the sequencing circuits continued to permit the application of highvoltage to the magnetron.

Accordingly, it is an object of this invention to provide a novelmagnetron power supply for use in a microwave oven;

[t is a further object of the invention to provide a microwave ovenpower supply which ensures proper operation of the magnetron; and

It is a still further object of the invention to provide a sequencingcircuit in the magnetron power supply which is simple, inexpensive,reliable, and which does not require a separate filament transformer.

Briefly, the invention encompasses a transformer having a primarywinding, a high voltage secondary winding and tapped low voltagesecondary winding; a magnetron includes an anode connected electricallyto ground potential and has a filament connected between the tap and oneside of the low voltage secondary winding; a first hot-wire relay has aheater coil connected across the low voltage secondary winding; a secondhot wire relay, substantially identical to the first, has a heaier coilconnected in series with the normally open contacts of the firsthot-wire relay across the low voltage secondary winding; a capacitor,the normally open set of contacts of the second hot-wire relay, and arectifier diode are connected in series across the high voltagesecondary winding of the transformer, with the start end of thatsecondary connected to the negative polarity terminal of the diode andsuch juncture being connected to ground, and, in addition, the positivepolarity diode terminal is connected to the electron-emitting element ofthe magnetron, the filament or cathode.

The foregoing and other advantages and features which are believed to becharacteristic of the invention both as to its organization and methodof operation together with further objects and advantages thereof arebetter understood from the following description considered inconnection with the accompanying drawing in which:

The FIGURE of the drawing illustrates by way of example one embodimentof the invention.

The preferred embodiment includes a transformer l which has a primarywinding 2, a high voltage secondary winding 5, and a tapped low voltagewinding 9 wound on an iron core 11. Transformer 1 is preferably of theconventional voltage regulating variety in which the primary andsecondary windings are loosely coupled together by winding them side byside on a common leg of the iron core 11 so that a leakage flux pathexists between the primary and secondary windings and the othercore-legs. Such leakage flux is increased to meet the needs ofparticular circuits in the conventional manner by the inclusion ofmagnetic material termed a shunt leg betweenthe primary and secondarywindings and the other core legs which acts as a magnetic shunt.Secondary winding 5 includes the start terminal 4 and end terminal 3.The term start refers to the windings of the secondary wound closest tocore 11.

Primary winding 2 is connected in series with a circuit breaker 12, thedoor interlock switch 13, timer switch 14 to a source of l 15 volt,60-cycle live current.

A magnetron 17 includes an anode l8 and a filament 19. Filament 19includes two leads 20 and 21, and an electronemissive coating whichpermits the filament to function as the cathode. The anode is connectedto a common electric potential termed the chassis ground represented bythe symbol 22. The two filament leads are connected between the tap 6 ofthe low voltage windings and end 7 of low voltage secondary winding 9.Suitably the voltage which appears between leads6 and 7 is the voltagespecified for filament 19.

A first hot-wire relay includes a heater winding 24 and a set ofnormally open contacts 25. The heater winding is connected between lead8 and lead 7 of low voltage secondary winding 9. This type of hot-wirerelay is commercially available and is illustrated in the productcatalogue of the King- Seely Thermos Company.

The contacts 25 of the first hot-wire relay are connected between lead 7and lead 8 of low voltage winding 9 in series with the heater coil 26 ofa second hot-wire relay. The second hot-wire relay is of substantiallyidentical construction as the first hot-wire relay and includes a secondset of normally open contacts 27.

The positive polarity terminal of a rectifier diode 28 is connected toone side of normally open contacts 27, and to filament lead 21. Thenegative polarity diode terminal is connected to the chassis ground 22.A high value resistor 29 is placed across contacts 27 to form arelatively high resistance bypass. Start lead 4 of high voltagesecondary winding 5 is connected to chassis ground. A capacitor 30 isconnected between the end lead 3 and to the other contact of normallyopen contacts 27 in the second hot-wire relay.

In normal operation the oven door, not illustrated, is closed. Hence,interlock 13 is closed. Circuit breaker 12 is-closed. The timer isoperated by the operator and timer contacts 14 are closed to permitcurrent to flow through primary winding 2 for the predetermined cookinginterval as determined by the setting of a conventional timer mechanism.The primary voltage ll5 volts is stepped up in transformer l and appearsas ahigh voltage of about 2,200 volts across secondary windings. Theprimary voltage is additionally stepped down to a low voltage of about 9volts which appears across'winding 9. The filament voltage for magnetron17 is about 4.6 volts and appears between the tap 6 and 7 of winding 9.

Filament current flows in a path from secondary winding 9, the lead 7,the filament 20 and 21 and back to the transformer winding through tap6.

Concurrently current flows from lead 8 through heater coil 24 and backthrough lead 7. The voltage which appears between leads 7 and 8 isgreater than the filament voltage. With the hot-wire relays commerciallyavailable a voltage greater than the filament voltage is necessary'foroperation. However, it is understood that with nonstandard hot-wirerelays or in an oven with a magnetron which requires a larger filamentvoltage then an intermediate tap would be unnecessary on winding 9 andthe relay heater 24 and the second hotwire relay circuit with the lead20 from filament 21 being connected to the winding end and could beplaced directly across filament winding 9.

The hot-wire relay is of a construction in which the normally opencontacts are supported by high expansion wires. These are temperatureresponsive. Heat sufficiently generated in wire heater 24 causes thewire to expand. With the requisite degree of expansion and, accordingly,after the lapse of a predetermined time interval contacts 25 close. Thehot-wire relay is selected with the criteria that the filament 19 isheated to proper levels during the aforementioned time interval.

Closure of contacts 25 permits current to flow from lead 7 to lead 8through heater coil 26 of the second hot-wire relay. Operation ofhot-wire relaycontacts 27 is similar to that of 25, however, since veryhigh voltages are present across contacts 27 current flows prematurelydue to arcing across the contacts prior to actual contact closure. Byitself the second hot-wire relay cannot be used for providingsignificant or consistent time delays necessary for'proper warming ofthe magnetron filament.

Considering closure of contacts 27 a circuit is completed in which thesecondary winding 5, capacitor 30, diode 28, and magnetron 17 appears asthe familiar half-wave voltage doubler circuit in which thehalf-wave-rectifying properties of the magnetron, here the power supplyload, is utilized as one of the power supply rectifiers. This rectifierarrangement is described in the Radio Engineers Handbook, Terman, i943and in U.S. Pat. No. 3,265,850. Thus, the voltage generated between theend of capacitor 30 and chassis ground is approximately twice thatacross high voltage winding 5. On one-half cycle of AC diode 28 conductsand charges capacitor 30 in one direction. On the other half cycle thepolarity across winding reverses placing that voltage in series withthat across the capacitor and magnetron l7 conducts current of thatvoltage on this half cycle. This process is repeated in each half cycle.The magnetron generates microwave energy which is conducted away in aconventional manner to an oven cavity, not illustrated. Reference isagain made to resistor 29. This resistor absorbs some of the energy thatwould otherwise be dissipated in an arc across contacts 27 prior toclosure. Additionally, when the power supply is off, resistor 29 bleedsoff any charge retained in capacitor 30.

At the completion of the timing interval timer contacts 14 open andinterrupt the flow of current to the transformer. Likewise, relays 24and 26 cool and restore their contacts to the normally open condition.

In addition to the foregoing sequence of operation provided by thehot-wire relays should the filament winding 9 be shorted or,alternatively, become highly resistive to a degree insufiicient to causeoperation of circuit breaker 12, the current through relay heater coil24 is reduced. Accordingly, the hot-wire relay coil 24 does not generatesufficient heat and cannot close its contacts 25. Hence, the highvoltage cannot be applied to the magnetron. In this instance failure ofthe most expensive oven component, the magnetron, is not only avoidedbut with this indication the cause of the trouble attributed to thefilament winding of the transformer can be quickly located and remedied.

It is understood that the foregoing description and illustration arepresented as illustrative of the invention and not by way of limitation.Inasmuch as numerous modifications and equivalents suggest themselves tothose skilled In the art it is expressly understood that the inventionis to be broadly construed within the spirit and scope of the appendedclaims.

What I claim is:

l. A power supply for use in a microwave oven comprising:

a. A magnetron for generating microwave energy, said magnetron includingan anode and a first and second filament leads;

b. a power transformer having a high voltage secondary winding with astart and end lead, and a tapped low voltage secondary winding withfirst, second, and top leads, and a primary winding;

c. a rectifier diode having a positive polarity terminal and a negativepolarity terminal;

. a capacitor; 7

e. a first hot-wire relay which includes a first heater winding and afirst set of normally open contacts, said contacts responsive to apredetermined high temperature generated by said heater winding for apredetermined period of time for operating to the closed position;

. a second hot-wire relay which includes a second heater winding and asecond set of normally open contacts, said contacts responsive to apredetermined high temperature generated by said heater winding for apredetermined period of time for operation to the closed position; I

g. means connecting said first filament lead to said top lead of saidlow voltage secondary winding;

h. means connecting said second filament lead to said first lead of saidlow voltage secondary winding;

i. means connecting said anode, said negative polarity diode terminal,and said start lead of said high voltage secondary winding to a commonelectrical potential or ground;

j. means connecting said positive polarity diode terminal,

said second filament lead and one side of said second normally opencontacts electrically in common;

k. means connecting said capacitor between said end lead of said highvoltage secondary winding and the other side of said second normallyopen contacts;

1. means connecting said first heater winding between said first andsecond leads of said low voltage secondary wind- 'ing; and

m. means connecting said first normally open contacts and said secondheater winding in series between said first and second leads of said lowvoltage secondary winding.

2. The invention as defined in claim 1 further comprising a highresistance connected across said second normally open contacts.

1. A power supply for use in a microwave oven comprising: a. A magnetronfor generating microwave energy, said magnetron including an anode and afirst and second filament leads; b. a power transformer having a highvoltage secondary winding with a start and end lead, and a tapped lowvoltage secondary winding with first, second, and top leads, and aprimary winding; c. a rectifier diode having a positive polarityterminal and a negative polarity terminal; d. a capacitor; e. a firsthot-wire relay which includes a first heater winding and a first set ofnormally open contacts, said contacts responsive to a predetermined hightemperature generated by said heater winding for a predetermined periodof time for operating to the closed position; f. a second hot-wire relaywhich includes a second heater winding and a second set of normally opencontacts, said contacts responsive to a predetermined high temperaturegenerated by said heater winding for a predetermined period of time foroperation to the closed position; g. means connecting said firstfilament lead to said top lead of said low voltage secondary winding; h.means connecting said second filament lead to said first lead of saidlow voltage secondary winding; i. means connecting said anode, saidnegative polarity diode terminal, and said start lead of said highvoltage secondary winding to a common electrical potential or ground; j.means connecting said positive polarity diode terminal, said secondfilament lead and one side of said second normally open contactselectrically in common; k. means connecting said capacitor between saidend lead of said high voltage secondary winding and the other side ofsaid second normally open contacts; l. means connecting said firstheater winding between said first and second leads of said low voltagesecondary winding; and m. means connecting said first normally opencontacts and said second heater winding in series between said first andsecond leads of said low voltage secondary winding.
 2. The invention asdefined in claim 1 further comprising a high resistance connected acrosssaid second normally open contacts.